The search preference _prefer_node allows specifying a single node to
prefer when routing a request. This functionality can be enhanced by
permitting multiple nodes to be preferred. This commit replaces the
search preference _prefer_node with the search preference _prefer_nodes
which supplants the former by specifying a single node and otherwise
adds functionality.
Relates #18872
Just like specifying `?preference=_primary`, this adds the ability to
specify `?preference=_replica` or `?preference=_replica_first` on
requests that support it.
Resolves#12222